In Canadian Pat. No. 927,041 there is described the preparation of phenol-formaldehyde resins from mole ratios of formaldehyde to phenol greater than 1:1 by a two-step procedure in an aqueous reaction medium containing a metal carboxylate catalyst therefor. In the first exothermic step, the reaction mixture is maintained at a temperature of about 60.degree. to about 80.degree. C., and in the second step a temperature of 90.degree. to 100.degree. C. is used, typically about 90.degree. C. While the procedure outlined in this prior art is said to be applicable to a wide range of mole ratios of formaldehyde to phenol of from 1:1 to 3:1, the specific example thereof is limited to a mole ratio of 1.5:1.
The resins produced by the two-step procedure of this prior art typically have a low viscosity at room temperature and are unsuitable for many adhesive applications, such as rice husk board manufacture. By prolonging the cooking procedure of the second stage of the resin-forming process, it is possible to provide a resin of high viscosity at room temperature (greater than about 50,000 cps at 75.degree. F.) which is suitable for rice husk board manufacture, as described in more detail in our U.S. Pat. No. 3,850,677.
The high viscosity resins provided by this modification of the procedure of Canadian Pat. No. 927,041 require heating to decrease their viscosity for application to the substrate. Acids typically are used to accelerate the rate of cure of the resins. However, the resins cure rapidly in the presence of acid at the elevated temperatures required for application to the substrate, leading to separate application of resin and acid to the substrate.
This mode of application is haphazard at best since proper contact of acid and resin on the substrate surface is by no means assured, and this leads to the use of greater quantities of acid than otherwise would be required, resulting in uneconomic chemical use and often impaired product appearance.
In Canadian Pat. No. 927,041, the resin produced by the procedure described therein is described as containing a preponderance of benzyl ether linkages between phenolic rings, mainly at ortho positions relative to the phenolic hydroxyl group and exhibiting a characteristic infra-red spectrum exhibiting large absorption at wave numbers of 1230 cm.sup.-1, 1050 cm.sup.-1 (1060 cm.sup.-1) and 1010 cm.sup.-1, and are reported to possess very long shelf lives at room temperature.
A further prior art patent, U.S. Pat. No. 3,485,797, describes the preparation of phenol-formaldehyde resins by a procedure involving anhydrous conditions using para-formaldehyde and phenol and water-immiscible solvents such as toluene to remove the water of condensation. This procedure uses relatively expensive catalysts.
In both pieces of prior art, the addition of acid to the resin is described for the purpose of accelerating cure of the resin.